In electronic devices and the like, a switching power supply is used to supply power to a load. For example, a DC-DC converter for converting a direct-current voltage into another direct-current voltage is used. A comparator type DC-DC converter is known as a DC-DC converter capable of responding to a sudden change of a load at a high speed (for example, see, Japanese Laid-Open Patent Publications No. 2010-51073).
FIG. 15 illustrates an example of a comparator type DC-DC converter. The illustrated DC-DC converter 4 includes a converter unit 5 and a control circuit 6. The converter unit 5 includes transistors T11, T12, a coil L11 and a capacitor C11.
A comparator 80 in the control circuit 6 receives a reference voltage VR11 and a feedback voltage VFB corresponding to the output voltage Vo. The feedback voltage VFB represents a voltage obtained by superposing an alternating-current component of an output voltage Vo on a divided voltage obtained by dividing the output voltage Vo by resistors R11, R12 through a capacitor C12 coupled in parallel to the resistor R11. The comparator 80 compares the feedback voltage VFB and the reference voltage VR11 and outputs an output signal S11 of a level corresponding to the comparison result to a set terminal S of an RS-flip-flop (RS-FF circuit) 81. An oscillator 82 outputs a clock signal CLK having a constant frequency to a reset terminal R of the RS-FF circuit 81.
The RS-FF circuit 81 is reset in response to an H-level clock signal CLK and outputs an L-level output signal S12. In response to the L-level output signal S12, a driver circuit 83 outputs H-level control signals DH, DL to turn off the transistor T11 and turn on the transistor T12. At this time, a switch circuit SW11 is turned off in response to the L-level output signal S12 output from the RS-FF circuit 81. Then, the capacitor C12 is charged according to a current I11 supplied from a current source 84, wherefore the reference voltage VR11 increases with a constant slope (=I11/C12) from a standard voltage VR0.
When the reference voltage VR11 becomes higher than the feedback voltage VFB, the comparator 80 outputs an H-level signal S11. In response to the H-level signal S11, the RS-FF circuit 81 is set and outputs an H-level output signal S12. Then, the driver circuit 83 outputs L-level control signals DH, DL to turn on the transistor T11 and turn off the transistor T12.
As described, in the comparator type DC-DC converter 4, the feedback voltage VFB corresponding to the output voltage Vo and the reference voltage VR11 are constantly compared by the comparator 80 and the main transistor T11 is immediately switched according to the comparison result. Thus, the comparator type DC-DC converter 4 responds to a sudden change of a load at a high speed.
In DC-DC converters of recent years, further miniaturization is required with a movement toward higher frequency characteristics. To meet such a miniaturization requirement, a multilayer chip coil is used as the coil L11. However, the multilayer chip coil generally has a poor direct-current superposition characteristic. Thus, if a current flowing in the coil L11 varies due to a variation of an output current Io, an inductance value of the coil L11 accordingly varies. Then, there arises a problem that frequency characteristics (frequency band and phase margin) of the DC-DC converter change due to the variation of the inductance value. Particularly, when a current flowing in the coil L11 increases, the inductance value of the coil L11 decreases and the frequency band spreads toward a high-frequency side, wherefore there arises a problem of reducing a phase margin.